1. Field of the Invention
This invention relates generally to a glass pressure-vessel, and particularly, but not exclusively, to a glass pressure-vessel for a cathode ray tube of a flat type, the tube being sealed up for keeping its internal pressure low.
2. Description of the Prior Art
Disclosed, for example, in Japanese Laid Open Patent No. 2-289444 is a glass pressure-vessel suitable for use with a TV of a thin type. The pressure vessel is produced by such a process as to heat and bend a glass plate to obtain a concave glass and, then, to bond the concave glass and a glass back-plate together with glass frit to obtain a closed vessel. It is noted that many electron guns are accommodated in the closed vessel.
However, in such pressure vessel, the central part of the glass back plate is deformed to be inwards convex due to low internal pressure thereof, so that the pressure withstanding strength of its bonded part decreases due to tensile stress produced in the bonded part. Particularly, in a glass pressure-vessel for a large-sized image display, in which the area of its flat portion is large, the withstanding strength decreases very much, so that the reliance upon the sealing ability of the glass pressure-vessel is placed low.
Moreover, the glass pressure-vessel accommodating the electron guns and so forth necessary for displaying images are usually sealed under low pressure and in a heating state to keep its interior and the articles therein dry, so that thermal stress is produced in the concave glass and glass back-plate, and particularly in the glass pressure-vessel having the large flat portion, the excessively high stress is produced in the sealed portion, so that the sealed portion is apt to be damaged.
In addition, the glass pressure-vessel for a cathode ray tube has a problem that it is undesirably colored (hereinafter, referred to "browning phenomenon"), since the electron beam being accelerated to about 10-30 kV is continuously applied to its inner surface.
Furthermore, the glass pressure-vessel for a cathode ray tube would have a high temperature and high voltage due to the continuous application of the electron beam. So that dielectric breakdown between the concave glass and the glass back-plate would be occurred, if the conventional concave glass with low electrical resistivity at high temperature are used for. Accordingly, such concave glass is not suitable for a cathode ray tube.